a) Field of the Invention
The present invention relates to an electrical power distribution installation and, more particularly, to apparatus and method for grounding and protecting the electrical power distribution installation such as a power station, sub-station, and/or a power generation installation in normal service.
b) Description of the Related Art
FIG. 1 shows a skeleton diagram representing a previously proposed electric power distribution installation.
As shown in FIG. 1, two main transformers MT1 and MT2 are used to step down respectively corresponding power supply voltages from respective feeders connected to receive powers L1 and L2 into main common bus bars BUS-A and BUS-B, each of 11 kV class.
Respective loads LOADs receive the stepped down power supply voltages from the respectively connected main common bus bars BUS-A and BUS-B.
An interconnecting circuit breaker 52AB which is normally closed serves to interconnect one of the main common bus bars BUS-A with the other main common bus bar BUS-B. Each main common bus bar BUS-A and BUS-B supplies the electric power to a corresponding load connected to a low-voltage bus bar via a corresponding transformer T11 and T21.
The other main bus bar BUS-B is connected directly to a generator common bus bar BUS-G via an interconnection feeder without passing through a step-down or step-up transformer.
The generator common bus bar BUS-G is connected to four generators G1, G2, G3, and G4. Each generator G1 through G4 can be disconnected from the generator common bus bar BUS-G by means of a corresponding circuit breaker 52G1, 52G2, 52G3, and 52G4. Each main transformer MT1 and MT2 can be disconnected from the corresponding main common bus bars BUS-A and BUS-B by means of corresponding circuit breakers 52S1 and 52S2. It is noted that each load can be disconnected from each corresponding main common bus bar BUS-A and BUS-B by means of corresponding circuit breakers as shown in FIG. 1.
A grounding device is installed in such a power distribution installation as shown in FIG. 1.
That is to say, the grounding device includes: a neutral grounding resistors NGR-1 and NGR-2, each grounding a neutral point of a star connection of a secondary winding of the corresponding main transformer MT1 or MT2; and a neutral grounding resistor grounding each neutral point of the generators G1 through G4 via a corresponding (vacuum) switch VS1, VS2, VS3, and VS4.
In the previously proposed grounding system described in the BACKGROUND OF THE INVENTION, suppose that a rating of each grounding resistor NGR-1, NGR-2, and NGR-G is 300 A-10 s.
If all of the grounding resistors NGR-1, NGR-2, and NGR-G are operated, a 900 A resistance grounding system can be constituted in the power system of FIG. 1.
However, if a failure in the feeder connected to the main transformer MT1 occurs so that the current breaker 52S1 interconnected to the secondary winding of the main transformer MT1 trips, the grounding device, i.e., the neutral grounding resistor NGR-1 is disconnected from the power system. Consequently, the grounding system is changed to a 600 A resistance grounding system in the power system.
In addition, if the failure occurs in the main transformer MT1 and, during a repair or exchange (replacement) of the transformer MT1, a failure in the generator common bus bar BUS-G occurs, the grounding device, i.e., the neutral grounding resistor NGR-G is also disconnected (separated) from the power system so that the grounding system is changed to a 300 A resistance grounding system.
If an electrical service interruption in a primary side of the main transformers MT1 and MT2, only the generators G1 through G4 need to continue to operate the power system through the 300 A resistance grounding system.
In the way described above, the change of the resistance grounding system due to the separation of each or any of the neutral grounding resistors from the power system causes changes in a ground fault detection sensitivity and a ground fault detection time in a protective relay system and often allows a coordinate protection in the power system not to be maintained.
It is therefore an object of the present invention to provide an electrical power distribution installation having an improved resistance grounding system and a protection system which are stable against a failure occurrence in an internal power system with no influence or small influence of a coordinate protection.
According to one aspect of the present invention, there is provided with an electric power distribution installation comprising: at least one main common bus bar; a plurality of main transformers, each main transformer being configured to step down a received power supply voltage and to supply the stepped down power supply voltage to a load via the main common bus bar; a plurality of power generators, each power generator being connected to the main common bus bar; a first grounding device including at least one grounding transformer connected to the main common bus bar, a neutral point of the grounding transformer being grounded in a form of a predetermined low impedance grounding; and a second grounding device connected to each neutral point of the power generators to always ground each neutral point directly in a form of a predetermined high resistance grounding.
According to another aspect of the present invention, there is provided with a method applicable to an electric power distribution installation, the electrical power distribution installation comprising: at least one main common bus bar; a plurality of main transformers, each main transformer being configured to step down a received power supply voltage and to supply the stepped down power supply voltage to a load via the main common bus bar; and a plurality of power generators, each power generator being connected to a generator common bus bar and the generator common bus bar being connected to the main common bus bar, and the method comprising: providing a first grounding device including at least one grounding transformer connected to the main common bus bar, a neutral point of the grounding transformer being grounded in a form of a predetermined low impedance grounding; and providing a second grounding device connected to each neutral point of the power generators to always ground each neutral point directly in a form of a predetermined high resistance grounding.
In the previously proposed electrical power distribution installation shown in FIG. 1, the four generators G1 through G4 are enabled to be a parallel operation and their one neutral point is grounded via their corresponding circuit breaker (switch) VS1 through VS4 by means of the single neutral grounding resistor NGR-G.
Reasons for adopting such a grounding method as described above and problems occurring therein will be described below.
(a) When each neutral point of the four generators G1 through G4 is short-circuited without an impedance, a zero-phase voltage component of a generated voltage of each generator G1 through G4 generates a zero-phase current at a corresponding neutral point so that an overheat of the respective generators G1 through G4 occurs.
To eliminate such a deficiency as described above, with any one of the vacuum switches VS1 through VS4 turned on in the normal service, only one of the four generators G1 through G4 which is connected to the turned-on vacuum switch is connected to the grounding resistor NGR-G.
(b) Each generator G1 through G4 is required for a regular inspection. If one of the four generators G1 through G4 which is connected to the grounding resistor NGR-G undergoes the inspection, it is necessary to switch the connection of the grounding resistor to another of the generators with no interruption of the power supply. At this time, any two of the four vacuum switches VS1 through VS4 are temporarily turned on in parallel to each other so as to prevent the grounding resistor from being separated from the generators even at an instantaneous time.
In addition, if a failure occurs in the feeder related to one of the generators G1 through G4 which is connected to the grounding resistor, it is necessary to connect automatically the grounding resistor to the neutral point of any other generator before the failed generator is separated from the grounding resistor.
For such a reason as described above, an automatic switching device or a normal operation by an operator is required which carries out a switching of the connection of the grounding resistor with any one of all of the generators always connected to the grounding resistor and with no occurrence in an instantaneous separation of the grounding resistor from the other of the generators during the inspection or during the failure occurrence.
(c) If, during the parallel operation of the generators, the operation of any one of the generators has been stopped due to the failure in the feeder connected to the corresponding generator, the circuit breaker remains in an off state which is installed between the common bus bar and the generator when the failed generator is recovered, the recovered generator is operated under a no-load driving condition. After the generated voltage is established, the connected vacuum switch is turned on to connect the recovered generator to the corresponding common bus bar.
If the ground fault occurs during the no-load operation of the recovered generator, a resonance phenomenon often occurs due to a reactance in a ground fault point and a capacitance between the ground and the feeder connected to the recovered generator. This resonance phenomenon causes a high voltage surge so that a dielectric breakdown may occur in any generator and/or a switchboard.
It is another object of the present invention to provide grounding method and protection method for the electric power distribution installation which require no switch of the connection of the grounding resistor to any other of the generators and assure the protection of the generators in the power distribution installation.
According to a still another aspect of the present invention, there is provided with a method applicable to an electric power distribution installation, the electrical power distribution installation comprising: at least one main common bus bar; a plurality of main transformers, each main transformer being configured to the stepped down power supply voltage to a load via the main common bus bar; and a plurality of power generators, each power generator being connected to a generator common bus bar and the generator common bus bar being connected providing a first grounding device including at least one grounding transformer connected to the main common bus bar, a neutral point of the grounding transformer being grounded in a form of a predetermined low impedance grounding; providing a second grounding device connected to each neutral point of the power generators to always ground each neutral point directly in a form of a predetermined high resistance grounding; providing a first protecting device including a ground over-current relay to a secondary feeder connecting a secondary winding of each main transformer to the main common bus bar and being enabled to separate the corresponding one of the main transformers from the main common bus bar; providing a second protecting device including a ground directional relay interposed in a feeder connected to the grounding transformer to detect a failure in the feeder connected to the grounding transformer; and providing a third protecting device including a ground directional relay to detect a ground fault in each corresponding power generator to be enabled to separate the ground fault occurring power generator from the main common bus bar.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.